The present invention relates to thread cutting apparatus in general, and more particularly to improvements in apparatus for making threaded tapping holes in pieces of metallic or other materials.
German Auslegeschrift No. 1,176,450 of Hafner discloses a manually rotatable fluted tap which can be used to cut threads in arcuate internal projections of workpieces, namely, in projections which extend inwardly from the surfaces bounding the so-called tapping holes of the workpieces. The tap is inserted into the hole in such a way that each of its externally threaded ribs extends into one of the recesses between the internal projections of the workpiece, and the tap is thereupon rotated through an angle such that each rib cuts threads in one of the projections, i.e., that each rib advances from a preceding recess into the next-following recess of the surface bounding the tapping hole. An advantage of such mode of threading tapping holes is that the tapping is completed in response to relatively small angular displacement of the tap, i.e., in response to rotation of the tap through less than one full revolution, and also that the tap can be readily withdrawn from the tapping hole upon completion of the thread cutting operation because there is no need to rotate the tap in reverse. However, the German publication of Hafner does not disclose or show any apparatus which can be used to locate, turn and/or withdraw the tap relative to and with reference to a workpiece.